Docker Multiarch Builds Quick and Easy

2019/11/09

As you all known from my things page, I run a multiarch cluster of nodes to run a variety of my services. More often than not, service images provided by maintainers only support x86 and x86_64 architectures. As a consequence of this, I have to self-build my service images.

When it was just building for one type of architecture (i.e. arm32) for my Odroid XU4, it was not so bad. Once I added another node, my arm64 Odroid-N2, it started to become a hassle to manually author custom Dockerfiles to build for each architecture by hand. Manual building was a cumbersome process that required me to ssh into each node and installing required dependencies to build the service image on the node with said supported architecture. You can imagine how slow this process is on an ARM SBC.

Instead, to streamline the process, my setup involves using QEMU emulation, binfmt and some bash scripting. The idea is simple, instead of building the service image on the target device with target architecture, just build on a powerful x86_64 machine with the QEMU emulator. To solve the challenge of invoking the QEMU emulator manually, I use binfmt_misc kernel capability to enable arbitrary execution of file formats. This capability is really powerful since the kernel inspects the binary file format to look for its runtime signature and it then invokes the appropriate program to execute said file.

Setup

On Archlinux, installing qemu-user-static-bin includes a set of binfmt configurations for various architectures (i.e. armeb, arm, etc.) and a set of statically compiled QEMU emulators. It is important that the static binaries are used when building docker images for different architecture. When building for a target architecture, all programs (i.e. linkers and other build tools) run in the target architecture. Therefore, without the static binary, you will encounter an exec init error when building your image.

Contents of the Package:

qemu-user-static-bin /usr/
qemu-user-static-bin /usr/bin/
qemu-user-static-bin /usr/bin/qemu-aarch64-static
qemu-user-static-bin /usr/bin/qemu-aarch64_be-static
qemu-user-static-bin /usr/bin/qemu-alpha-static
qemu-user-static-bin /usr/bin/qemu-arm-static
qemu-user-static-bin /usr/bin/qemu-armeb-static
qemu-user-static-bin /usr/bin/qemu-cris-static
qemu-user-static-bin /usr/bin/qemu-hppa-static
qemu-user-static-bin /usr/bin/qemu-i386-static
qemu-user-static-bin /usr/bin/qemu-m68k-static
qemu-user-static-bin /usr/bin/qemu-microblaze-static
qemu-user-static-bin /usr/bin/qemu-microblazeel-static
qemu-user-static-bin /usr/bin/qemu-mips-static
qemu-user-static-bin /usr/bin/qemu-mips64-static
qemu-user-static-bin /usr/bin/qemu-mips64el-static
qemu-user-static-bin /usr/bin/qemu-mipsel-static
qemu-user-static-bin /usr/bin/qemu-mipsn32-static
qemu-user-static-bin /usr/bin/qemu-mipsn32el-static
qemu-user-static-bin /usr/bin/qemu-nios2-static
qemu-user-static-bin /usr/bin/qemu-or1k-static
qemu-user-static-bin /usr/bin/qemu-ppc-static
qemu-user-static-bin /usr/bin/qemu-ppc64-static
qemu-user-static-bin /usr/bin/qemu-ppc64abi32-static
qemu-user-static-bin /usr/bin/qemu-ppc64le-static
qemu-user-static-bin /usr/bin/qemu-riscv32-static
qemu-user-static-bin /usr/bin/qemu-riscv64-static
qemu-user-static-bin /usr/bin/qemu-s390x-static
qemu-user-static-bin /usr/bin/qemu-sh4-static
qemu-user-static-bin /usr/bin/qemu-sh4eb-static
qemu-user-static-bin /usr/bin/qemu-sparc-static
qemu-user-static-bin /usr/bin/qemu-sparc32plus-static
qemu-user-static-bin /usr/bin/qemu-sparc64-static
qemu-user-static-bin /usr/bin/qemu-tilegx-static
qemu-user-static-bin /usr/bin/qemu-x86_64-static
qemu-user-static-bin /usr/bin/qemu-xtensa-static
qemu-user-static-bin /usr/bin/qemu-xtensaeb-static
qemu-user-static-bin /usr/lib/
qemu-user-static-bin /usr/lib/binfmt.d/
qemu-user-static-bin /usr/lib/binfmt.d/qemu-aarch64-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-alpha-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-arm-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-armeb-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-cris-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-m68k-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-microblaze-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-microblazeel-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-mips-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-mips64-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-mips64el-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-mipsel-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-ppc-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-ppc64-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-ppc64abi32-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-ppc64le-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-s390x-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-sh4-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-sh4eb-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-sparc-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-sparc32plus-static.conf
qemu-user-static-bin /usr/lib/binfmt.d/qemu-sparc64-static.conf
qemu-user-static-bin /usr/share/
qemu-user-static-bin /usr/share/man/
qemu-user-static-bin /usr/share/man/man1/
qemu-user-static-bin /usr/share/man/man1/qemu-aarch64-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-aarch64_be-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-alpha-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-arm-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-armeb-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-cris-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-hppa-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-i386-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-m68k-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-microblaze-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-microblazeel-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-mips-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-mips64-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-mips64el-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-mipsel-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-mipsn32-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-mipsn32el-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-nios2-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-or1k-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-ppc-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-ppc64-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-ppc64abi32-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-ppc64le-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-riscv32-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-riscv64-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-s390x-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-sh4-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-sh4eb-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-sparc-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-sparc32plus-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-sparc64-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-tilegx-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-user-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-x86_64-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-xtensa-static.1.gz
qemu-user-static-bin /usr/share/man/man1/qemu-xtensaeb-static.1.gz

If you're not using Archlinux, you might want to find the equivalent package in your target distribution. This package is only installed on the host machine you use to build docker multiarch images.

Once this is installed, turn on binfmt_misc capability.

mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc
echo 1 > /proc/sys/fs/binfmt_misc/status

For a more permenant setup, echo "none /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0" | tee /etc/fstab and enable the systemd-binfmt service.

Now, you can build/run any Docker images of any architecture to your heart's content.

Just run docker build . -f [DOCKERFILE] -t [IMAGE_NAME]:[ARCH_TAG].

Multiarch Images

Docker now supports image manifests. Image manifest allows you to annotate additional details for specific tagged images. Remember to turn on experimental features for the Docker CLI tool at $HOME/.docker/config.json on the host machine where you are authoring the manifest. For example, supposed we want to build whoami for arm32, arm64 and x86, we can create an image manifest for whoami:

docker manifest create whoami:latest \
    whoami:arm32 \
    whoami:arm64 \
    whoami:x86

docker manifest annotate --arch arm --os linux whoami:latest \
    whoami:arm32
docker manifest annotate --arch arm64 --os linux whoami:latest \
    whoami:arm64
docker manifest annotate --arch x86 --os linux whoami:latest \
    whoami:x86
docker manifest push -p whoami:latest

This assumes that you've already build individual whoami images for each architecture. The -p flag for docker manifest push purges the manifest list locally, I found this to be useful as subsequent updates to the manifest require you to recreate the manifest from scratch and if a local copy exist, docker manifest will not run successfully. After pushing the manifest to the remote repository, you can now pull whoami:latest from target nodes of various architectures safely without needing to worry about including an [ARCH_TAG] for the image. For example, on my Odroid-XU4 which runs arm32 arch, it will automatically pull whoami:arm32 when I run docker pull whoami:latest.

References

  1. Architecture emulation containers with binfmt_misc
  2. Kernel Support for miscellaneous (your favourite) Binary Formats